WASHINGTON -- Presidential candidate Mitt Romney's tax returns may be more incomplete in their public release than was previously thought. Filings related to Romney's individual retirement account, already subject to press scrutiny, indicate that the presumptive GOP nominee should have filed a form accounting for unrelated business income taxes.

The unrelated business income tax is set up for tax-exempts like nonprofits and IRAs that engage in commercial activity. Not filing the form, known as a 990-T, may indicate that Romney's IRA funds are held by an offshore account in order to shield them from taxation.

Romney campaign spokesperson Andrea Saul did not return multiple requests for the 990-T form, nor did she answer repeated questions related to the matter.

From what Romney has disclosed, there is mounting circumstantial evidence that the IRA may hold offshore investments through what are known as "blocker corporations," which help him avoid paying taxes. Senator Max Baucus (D-Mont.) told The New York Times, "From what I have read about Governor Romney's tax returns, I think it raises very serious questions."

"We know Romney's IRA has Bain funds in it," explained Rebecca J. Wilkins, senior counsel for federal tax policy with the nonpartisan Citizens for Tax Justice. "Bain private equity funds are listed in those assets and those funds are located in the Cayman Islands."

Wilkins says that if Romney's IRA held those funds directly, instead of through a blocker corporation, a 990-T should have been filed. "Typically it doesn't even have to file a tax return," Wilkins says. "But if it has unrelated business income, then it has to pay the unrelated business income tax." And a 990-T would need to have been filed.

Romney's I.R.A. also appears to have invested in so-called blocker corporations in the Cayman Islands and elsewhere. U.S. pension funds, foundations, and even I.R.A.'s routinely use offshore blocker corporations to avoid something called the Unrelated Business Income Tax, which was designed to keep nonprofits from competing with ordinary companies in areas outside their core purpose: if you invest directly you get hit with the tax, but if you invest in a blocker, which then invests in the U.S. business, you escape it. Romney's I.R.A. appears to have employed this lawful escape route, and his campaign has used language suggesting that it has. But that would mean the Romney camp's claim that Mitt's tax consequences of investing via the Cayman Islands is 'the very same' as it would have been had he invested directly at home is simply not true."

If Romney had been operating a nonprofit, his use of the offshoring loophole would be similar to the sort of activities Sen. Chuck Grassley (R-Iowa) has long sought to shut down. The senator has made similar tax avoidances a priority since 2007.

Two years ago, Grassley and colleagues in the Senate were stunned to learn that the Boys and Girls Club of America held more than $50 million in offshore accounts and funds, including some held in the Cayman Islands. The organization had to admit that it had used the Caymans to avoid paying the unrelated business income tax.

Grassley's proposed legislation to close the loophole has yet to come up for a vote.

Jill Kozeny, a Grassley spokesperson, told HuffPost that the senator is not focused on IRAs that may employ offshoring. "This focus hasn't been on unlikely UBIT avoidance by pension funds, IRAs, or other tax-deferred retirement accounts since those funds and accounts are generally not using debt, such as tax-exempt bonds or real estate mortgages, to finance investments, which is what usually triggers UBIT for charities," she said via email.

Wilkins of Citizens for Tax Justice believes the loophole should be closed for IRAs. "Clearly it's an unintended loophole in the tax code," she says. "The reason they are making these investments offshore is to avoid paying the unrelated business tax."